I am trying to repair a 90 year-old toilet whose wall-mounted tank connects to the bowl without bolts. I can't find any apparent brand markings, although I did find the numbers "1 603H", 28, and 1916 on the tank lid. No online searches have turned up any results on what kind of toilet this is or what type of parts to use.

There's a pipe running from the bottom of the tank (via the flush mechanism) to the top of the bowl. As far as I can tell, the system relies on pressure and washers to ensure a tight seal, but the standard rubber washers I bought at home depot don't seem to be doing the trick. I've tried plumbers epoxy but it still leaks.

I've been unable to repair the flush mechanism without it leaking, and as a result am unable to even begin to figure out how to reconnect to the tank. I'd like to replace the whole flush mechanism, and pipe/connection to the bowl, but I don't know what parts to use, or even where to look.

The attached picture shows the pipe and connectors running from the tank to the bowl. The black is the dried out old washer that had fit in the bowl. Again - no bolts...

I'm feeling pretty good about having successfully reinstalled the toilet bowl and remounted the tank on the wall, so I'd love to finish the job. Anyone have any recommendations or ideas?

I would take the all the parts between the tank and the toilet to a plumbing supply house and speak with them about it. From the looks of it, it uses crush washers, basically, a rubber sleeve on the pipe gets inserted through the hole in the tank and toilet. When the nut is run down behind it, it crushes the rubber sleeve causing it to expand and seal the hole. This is a guess, mind you, from looking at the pictures you provided (thanks for that, very helpful! )

From the looks of it, it uses crush washers, basically, a rubber sleeve on the pipe gets inserted through the hole in the tank and toilet. When the nut is run down behind it, it crushes the rubber sleeve causing it to expand and seal the hole.

Thanks - that sounds exactly right. Now that I have a name for the washer I can ask around.

Do you think I'll be able to readily find replacement parts? I'm hesitant to pull apart the flush mechanism (although it looks fairly standard) because I think I'll have to saw it off to remove it, as it is either one piece (unlikely) or rusted/welded/soldered in place.

Measure the approximate length, including the parts that penetrate through the tank and toilet. Measure the outer diameter of the tube. Take this info along with the picture to a supply house so that they can see what you're working with and talking about. You should be able to get parts, or at least new parts that will work. You will need to go to a plumbing supplier and not just a hardware store or big box.

¼" play seems like it is a bit excessive, but you might try filling the gap with a generous amount of plumber's putty. Put a bead around the hole on top before you insert the valve assembly, then fill the gap and a bead on the bottom before you install the washer and nut.
Jack